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Process Engineering Rules of Thumb

Quick reference guidelines and heuristics for process engineering design and troubleshooting.

Heat Exchangers

Quick guidelines for heat exchanger design and troubleshooting

  • Approach temperature: Minimum approach temperature should be 5-10°C (10-20°F) for liquid-liquid exchangers and 15-25°C (30-45°F) for gas coolers.
  • Tube velocity: Maintain 1-3 m/s (3-10 ft/s) for liquids and 15-30 m/s (50-100 ft/s) for gases to balance heat transfer and pressure drop.
  • Shell-side velocity: Target 0.3-1 m/s (1-3 ft/s) for liquids to minimize vibration and erosion.
  • Tube-side pressure drop: Design for 0.5-1 bar (7-15 psi) for most applications.
  • Shell-side pressure drop: Design for 0.2-0.5 bar (3-7 psi) for most applications.
  • Fouling detection: A 10-15% increase in pressure drop or a 10-15% decrease in heat transfer coefficient indicates significant fouling.
  • Maintenance interval: Plan for cleaning every 1-3 years depending on service.

Distillation Columns

Quick guidelines for distillation column design and operation

  • Reflux ratio: Operate at 1.1-1.3 times minimum reflux for economic operation.
  • Tray spacing: Use 450-600 mm (18-24 inches) for columns larger than 1 m (3 ft) in diameter.
  • Weir height: Typically 50-75 mm (2-3 inches) for most applications.
  • Downcomer area: Size for 10-15% of column cross-sectional area.
  • Vapor velocity: Design for 70-85% of flooding velocity.
  • Pressure drop: Expect 2.5-7.5 mm (0.1-0.3 inches) of water per tray.
  • Feed location: Introduce feed at the tray where the liquid composition most closely matches the feed composition.
  • Turndown ratio: Most columns can operate at 30-70% of design capacity.

Pumps

Quick guidelines for pump selection and operation

  • NPSH margin: Provide at least 0.5 m (1.5 ft) of NPSH margin above the required NPSH.
  • Efficiency: Centrifugal pumps operate most efficiently at 70-120% of best efficiency point (BEP).
  • Suction specific speed: Keep below 11,000 (metric) or 3,500 (US customary) to avoid cavitation.
  • Minimum flow: Maintain at least 25% of BEP flow to prevent overheating.
  • Parallel operation: Ensure pumps have similar head-capacity curves when operating in parallel.
  • Vibration: Vibration levels above 7 mm/s (0.28 in/s) RMS indicate a problem.
  • Temperature rise: Temperature rise across a centrifugal pump should not exceed 15°C (27°F).